Chemically sensitive sensor with lightly doped drains

ABSTRACT

A chemically sensitive sensor with a lightly doped region that affects an overlap capacitance between a gate and an electrode of the chemical sensitive sensor. The lightly doped region extends beneath and adjacent to a gate region of the chemical sensitive sensor. Modifying the gain of the chemically sensitive sensor is achieved by manipulating the lightly doped region under the electrodes.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 13/173,793 filed Jun. 30, 2011, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/361,403 filed on Jul. 3, 2010, each of which is incorporated herein by reference in its entirety.

BACKGROUND

Electronic devices and components have found numerous applications in chemistry and biology (more generally, “life sciences”), especially for detection and measurement of various chemical and biological reactions and identification, detection and measurement of various compounds. One such electronic device is referred to as an ion-sensitive field effect transistor, often denoted in the relevant literature as an “ISFET” (or pHFET). ISFETs conventionally have been explored, primarily in the academic and research community, to facilitate measurement of the hydrogen ion concentration of a solution (commonly denoted as “pH”).

More specifically, an ISFET is an impedance transformation device that operates in a manner similar to that of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and is particularly configured to selectively measure ion activity in a solution (e.g., hydrogen ions in the solution are the “analytes”). A detailed theory of operation of an ISFET is given in “Thirty years of ISFETOLOGY: what happened in the past 30 years and what may happen in the next 30 years,” P. Bergveld, Sens. Actuators, 88 (2003), pp. 1-20 (“Bergveld”), which publication is hereby incorporated herein by reference in its entirety.

Details of fabricating an ISFET using a conventional CMOS (Complementary Metal Oxide Semiconductor) process may be found in Rothberg, et al., U.S. Patent Publication No. 2010/0301398, Rothberg, et al., U.S. Patent Publication No. 2010/0282617, and Rothberg et al, U.S. Patent Publication 2009/0026082; these patent publications are collectively referred to as “Rothberg”, and are all incorporated herein by reference in their entirety. In addition to CMOS, however, biCMOS (i.e., bipolar and CMOS) processing may also be used, such as a process that would include a PMOS FET array with bipolar structures on the periphery. Alternatively, other technologies may be employed wherein a sensing element can be made with a three-terminal devices in which a sensed ion leads to the development of a signal that controls one of the three terminals; such technologies may also include, for example, GaAs and carbon nanotube technologies.

Taking a CMOS example, a P-type ISFET fabrication is based on a P-type silicon substrate, in which an N-type well forming a transistor “body” is formed. Highly doped P-type (P+) regions S and D, constituting a source and a drain of the ISFET, are formed within the N-type well. A highly doped N-type (N+) region B may also be formed within the N-type well to provide a conductive body (or “bulk”) connection to the N-type well. An oxide layer may be disposed above the source, drain and body connection regions, through which openings are made to provide electrical connections (via electrical conductors) to these regions. A polysilicon gate may be formed above the oxide layer at a location above a region of the N-type well, between the source and the drain. Because it is disposed between the polysilicon gate and the transistor body (i.e., the N-type well), the oxide layer often is referred to as the “gate oxide.”

Like a MOSFET, the operation of an ISFET is based on the modulation of charge concentration (and thus channel conductance) caused by a MOS (Metal-Oxide-Semiconductor) capacitance. This capacitance is constituted by a polysilicon gate, a gate oxide and a region of the well (e.g., N-type well) between the source and the drain. When a negative voltage is applied across the gate and source regions, a channel is created at the interface of the region and the gate oxide by depleting this area of electrons. For an N-well, the channel would be a P-channel (and vice-versa). In the case of an N-well, the P-channel would extend between the source and the drain, and electric current is conducted through the P-channel when the gate-source potential is negative enough to attract holes from the source into the channel. The gate-source potential at which the channel begins to conduct current is referred to as the transistor's threshold voltage VTH (the transistor conducts when VGS has an absolute value greater than the threshold voltage VTH). The source is so named because it is the source of the charge carriers (holes for a P-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel.

As described in Rothberg, an ISFET may be fabricated with a floating gate structure, formed by coupling a polysilicon gate to multiple metal layers disposed within one or more additional oxide layers disposed above the gate oxide. The floating gate structure is so named because it is electrically isolated from other conductors associated with the ISFET; namely, it is sandwiched between the gate oxide and a passivation layer that is disposed over a metal layer (e.g., top metal layer) of the floating gage.

As further described in Rothberg, the ISFET passivation layer constitutes an ion-sensitive membrane that gives rise to the ion-sensitivity of the device. The presence of analytes such as ions in an analyte solution (i.e., a solution containing analytes (including ions) of interest or being tested for the presence of analytes of interest), in contact with the passivation layer, particularly in a sensitive area that may lie above the floating gate structure, alters the electrical characteristics of the ISFET so as to modulate a current flowing through the channel between the source and the drain of the ISFET. The passivation layer may comprise any one of a variety of different materials to facilitate sensitivity to particular ions; for example, passivation layers comprising silicon nitride or silicon oxynitride, as well as metal oxides such as silicon, aluminum or tantalum oxides, generally provide sensitivity to hydrogen ion concentration (pH) in an analyte solution, whereas passivation layers comprising polyvinyl chloride containing valinomycin provide sensitivity to potassium ion concentration in an analyte solution. Materials suitable for passivation layers and sensitive to other ions such as sodium, silver, iron, bromine, iodine, calcium, and nitrate, for example, are known, and passivation layers may comprise various materials (e.g., metal oxides, metal nitrides, metal oxynitrides). Regarding the chemical reactions at the analyte solution/passivation layer interface, the surface of a given material employed for the passivation layer of the ISFET may include chemical groups that may donate protons to or accept protons from the analyte solution, leaving at any given time negatively charged, positively charged, and neutral sites on the surface of the passivation layer at the interface with the analyte solution.

With respect to ion sensitivity, an electric potential difference, commonly referred to as a “surface potential,” arises at the solid/liquid interface of the passivation layer and the analyte solution as a function of the ion concentration in the sensitive area due to a chemical reaction (e.g., usually involving the dissociation of oxide surface groups by the ions in the analyte solution in proximity to the sensitive area). This surface potential in turn affects the threshold voltage of the ISFET; thus, it is the threshold voltage of the ISFET that varies with changes in ion concentration in the analyte solution in proximity to the sensitive area. As described in Rothberg, since the threshold voltage V_(TH) of the ISFET is sensitive to ion concentration, the source voltage V_(S) provides a signal that is directly related to the ion concentration in the analyte solution in proximity to the sensitive area of the ISFET.

Arrays of chemically-sensitive FETs (“chemFETs”), or more specifically ISFETs, may be used for monitoring reactions—including, for example, nucleic acid (e.g., DNA) sequencing reactions, based on monitoring analytes present, generated or used during a reaction. More generally, arrays including large arrays of chemFETs may be employed to detect and measure static and/or dynamic amounts or concentrations of a variety of analytes (e.g., hydrogen ions, other ions, non-ionic molecules or compounds, etc.) in a variety of chemical and/or biological processes (e.g., biological or chemical reactions, cell or tissue cultures or monitoring, neural activity, nucleic acid sequencing, etc.) in which valuable information may be obtained based on such analyte measurements. Such chemFET arrays may be employed in methods that detect analytes and/or methods that monitor biological or chemical processes via changes in charge at the chemFET surface. Such use of ChemFET (or ISFET) arrays involves detection of analytes in solution and/or detection of change in charge bound to the chemFET surface (e.g. ISFET passivation layer).

Research concerning ISFET array fabrication is reported in the publications “A large transistor-based sensor array chip for direct extracellular imaging,” M. J. Milgrew, M. O. Riehle, and D. R. S. Cumming, Sensors and Actuators, B: Chemical, 111-112, (2005), pp. 347-353, and “The development of scalable sensor arrays using standard CMOS technology,” M. J. Milgrew, P. A. Hammond, and D. R. S. Cumming, Sensors and Actuators, B: Chemical, 103, (2004), pp. 37-42, which publications are incorporated herein by reference and collectively referred to hereafter as “Milgrew et al.” Descriptions of fabricating and using ChemFET or ISFET arrays for chemical detection, including detection of ions in connection with DNA sequencing, are contained in Rothberg. More specifically, Rothberg describes using a chemFET array (in particular ISFETs) for sequencing a nucleic acid involving incorporating known nucleotides into a plurality of identical nucleic acids in a reaction chamber in contact with or capacitively coupled to chemFET, wherein the nucleic acids are bound to a single bead in the reaction chamber, and detecting a signal at the chemFET, wherein detection of the signal indicates release of one or more hydrogen ions resulting from incorporation of the known nucleotide triphosphate into the synthesized nucleic acid.

Ion-sensitive metal oxide field effect transistors (ISFET) are known. The chemical reactions sensed by these types of transistors result in electrical signals that are very small in magnitude, and therefore may require amplification by additional circuitry to provide signal gain so the signal may be processed efficiently. The additional circuitry takes up real estate on the semiconductor substrate that may be used for additional sensor elements instead of amplification circuitry. It would be beneficial if a chemically sensitive sensor could have a modified gain to eliminate the need for the additional gain circuitry. The inventor recognized the benefits of the following described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-D illustrate the building of a semiconductor substrate according to an embodiment of the present invention.

FIG. 2A-D illustrate a semiconductor doped to provide lightly doped drains according to an embodiment of the present invention.

FIG. 3 illustrates a diagram of the capacitance generated by the respective doping regions of a chemically sensitive sensor in an embodiment of the present invention.

FIG. 4 illustrates an exemplary structure of a chemically sensitive sensor according to an embodiment of the present invention.

FIG. 5 illustrates another exemplary structure of a chemically sensitive sensor according to an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments provide a chemically sensitive sensor with modified gain. The chemically sensitive sensor may include a microwell, a floating gate terminal, a drain terminal, a source terminal and a pair of doped regions in a substrate. The microwell may accept a sample used in a chemical reaction. The floating gate may be electrically coupled to a gate electrode on the substrate. The drain terminal connection and the source terminal connection may be electrical terminals on the chemically sensitive sensor. The pair of doped regions in the substrate may each include a lightly doped region and a highly doped region. Each of lightly doped regions may extend beneath the gate electrode on the substrate and each of the highly doped regions may extend to couple respectively to the drain terminal and the source terminal.

Another embodiment may also provide a chemically sensitive sensor with modified gain. The chemically sensitive sensor may include a microwell, a floating gate terminal, a drain terminal, a source terminal, a pair of electrodes and a pair of doped regions in a substrate. The microwell may accept a sample used in a chemical reaction. The floating gate may be electrically coupled to a gate electrode on the substrate. The drain terminal connection and the source terminal connection may be electrical terminals on the chemically sensitive sensor. The pair of electrodes may be formed on the substrate and one electrode of the pair on either side of the gate electrode. One of the pair of doped regions may include a lightly doped region and a highly doped region, while the other one of the pair of doped regions may only include a highly doped region. The lightly doped region may extend beneath a respective one of the electrodes and the highly doped of each pair extends to couple respectively with the drain terminal or the source terminal.

An embodiment may also provide a method for building a chemically sensitive sensor according to an embodiment of the present invention. The method may forming a substrate with a first conductivity type of dopant. An epitaxial layer may be built using the same conductivity type dopant used to form the substrate, but made less dense than the dopant on the substrate. An electrode layer may be formed on the epitaxial layer formed from a different, second conductivity type of dopant than the first conductivity type of dopant used to form the substrate. The density of dopant on both the electrode layer and the substrate may be similar. The electrode layer may be masked and etched to produce gates and electrodes. A first lightly doped region may be created adjacent to one of the electrodes using a multidirectional implant technique, wherein the first lightly doped region is formed from a dopant of a conductivity type opposite the epitaxial layer dopant. Diffusion nodes may be produced that are self-aligned with the electrodes next to the gates, a first of said diffusion nodes contiguous with the first lightly doped region, from a dopant of a conductivity type similar to the gates, electrodes, and lightly doped region. A floating gate electrode, electrodes above the diffusion area, and contacts for electrodes may be formed by alternating layers of insulation, dielectric, conductive and metal layers.

FIGS. 1A-D illustrate the building of a semiconductor substrate according to an embodiment of the present invention. In this embodiment, the chemically sensitive sensor 100 may be fabricated on a polysilicon substrate 110 with a semiconductor doping, which in this example is a P or (P+)-type dopant, is formed as illustrated in FIG. 1A. As shown in FIG. 1B, an epitaxial layer (P-epi) 120 may be formed upon the P+ type substrate 110 from doping of similar conductivity type, i.e., P-type, as the P+ type substrate 110, but at a lesser density. Of course, other doping such as N-type doping may be used.

The area where the charge coupled sensor cells are going to be formed may be pre-doped at a dense doping level with a dopant having conductivity type opposite, i.e., N+, to that of the substrate (P+) 110 and epitaxial layer 120. In FIG. 1C, an N+ doping level 130 may be built over the P-type epitaxial layer 120 and the P+ type substrate 110. The N+ doping level 130 may be used within the charge coupled chemical sensor area. Using a mask and etching operation, gate(s) 133 and electrodes 134, 136 of the chemically sensitive sensor 100 may be formed in the pre-doped areas of the N+ doping levels 130 as shown in FIG. 1D. It will be understood by those of ordinary skill within the art that in the described embodiment, the doping levels can be reversed.

The above disclosed embodiment describes the fabrication of a transistor that may provide a first gain to any signal between the electrodes 134, 136 based on a signal applied to the gate electrode 133. The gain of the chemically sensitive sensor 100 may be modified by inserting additional doping material at locations within the substrate 110 in locations proximate to the electrodes 134, 136 or gate 133. The additional doping material may affect the capacitance of the transistor 100, which consequently modifies the gain of the transistor 100. The details of the gain modification will be explained in more detail with respect to FIG. 3.

FIG. 2A illustrates an approximate location of additional doping in an epitaxial layer adjacent to a gate electrode according to an embodiment of the present invention. The device of FIG. 2A may have a substrate structure similar to that of FIG. 1C. The chemically sensitive sensor 200 may include an epitaxial layer 220 of a first conductivity type of dopant (i.e., P-type), a gate electrode 215 formed from a second conductivity type of dopant (i.e., N-type), and lightly doped regions 223, 225. The lightly doped regions 223, 225, also called lightly doped drains (LDD), may be of the same of conductivity type of dopant (i.e., P-type) as the gate electrode 215. The gain of the chemically sensitive sensor 200 may be modified by the injection of a less dense dopant in the lightly doped regions 223, 225 than the gate electrode 215. The lightly doped regions 223, 225 may be formed by using, for example, a multidirectional injection technique to inject the less-dense dopant into the lightly doped regions 223, 225 beneath the gate electrode 215. Of course, other techniques may also be used. Highly doped regions for the source and drain terminals of the transistor may be added either before or after the lightly doped regions. FIG. 2B illustrates the placement of diffusion nodes 223, 225, which may be highly doped regions, for coupling to the source and drain terminals of the chemically sensitive sensor 220 with respect to the gate electrode 215 and the lightly doped regions 223, 225. Using known doping techniques, the highly doped regions 223, 225 may be built with a high density dopant of the same conductivity type of dopant as the gate electrode 215.

FIGS. 2C and 2D illustrate an alternative embodiment for a chemically sensitive sensor that has a modified doping. FIG. 2C illustrates a chemically sensitive sensor 202 that in addition to a gate 210 may also have electrodes 214, 216. In FIG. 2C, the gates 210 and the electrodes 214, 216 of the chemically sensitive sensor 202 can be masked to enable multidirectional doping implants to be inserted adjacent to respective electrodes 214, 216 to create lightly doped regions 227, 229, or lightly doped drains (LDD). The LDD 227 and 229 may be formed from a conductivity type opposite the epitaxial layer, i.e., N+. In the illustrated embodiment, the LDD 227, 229 may be formed within the chemically sensitive sensor 202 beneath the electrodes 214, 216. Alternatively, the LDD 227, 229 may be implanted using alternate implanting methods as are known in the art.

A photo resist layer can be used to mask desired areas and form diffusion nodes 235, 237 that may be self aligned with the electrodes 214, 216 next to the gates 210 and contiguous with the LDDs 227, 229. The diffusion areas 235 and 237 may be formed from a conductivity type similar to the gates 210, electrodes 214, 216 and LDDs 227, 229, and opposite from the conductivity type of the epitaxial layer 221. In FIG. 2D, the N+ diffusion nodes 237 and 239 may be highly doped regions and may be formed using any conventional technique known within the semiconductor arts. In addition, other nodes may also be formed. The lightly doped region 227, 229 may be doped at a dopant density level that is less than the dopant density level of the diffusion nodes 237 and 239.

FIG. 3 is a diagram illustrating capacitances associated with the respective doping regions of a chemically sensitive sensor according to an embodiment of the present invention. The illustrated chemically sensitive sensor 300 may include a gate electrode 384 and diffusion areas 391 and 395 that are built on an epitaxial layer 397. The doping of the gate electrode 384 and diffusion areas 391, 395 is shown as N-type doping while the epitaxial layer 397 is shown as P-type. Of course, the doping may be reversed. The diffusion areas 391 and 395 may be doped at a high density. The diffusion area 391 may contact a source terminal of the chemically sensitive sensor 300 and the diffusion area 395 may contact a drain terminal of the chemically sensitive sensor 300. Of course, the source terminal and the drain terminal may be interchanged. The gate electrode 384 may connect to a floating gate (not shown) that will provide a signal. Depending upon the magnitude of the signal from the floating gate, the channel 396 may be induced to conduct according to known transistor principles. A signal on the drain diffusion area 395 may pass through the channel 396 to the source diffusion area 391. In addition, due to the fabrication of the sensor 300, a gate-to-drain capacitance Cgd may be present between the gate electrode 384 and the drain diffusion area 395. Similarly, a gate-to-source capacitance Cgs may be present between the gate electrode 384 and the source diffusion area 391. The values of these capacitances Cgd and Cgs may influence the signal gain of the sensor 300. The capacitance Cgd and Cgs may be the result of the junction area between the doping of the diffusion areas 391 and 395 and the gate electrode 384. A portion of the capacitance Cgd and Cgs may be attributed to parasitic capacitance at the junction of the diffusion area and the gate electrode 384. The amount of parasitic capacitance may be adjusted by adding lightly doped regions 394 and 392 to the drain and source diffusion areas 395 and 391, respectively. The lightly doped regions 394 and 392 may be doped at a lower density than the highly doped diffusion areas 391 and 395, but may add additional area at the junction of the diffusion areas 395 and 391 with the gate electrode 384. Due to the additional area, additional parasitic capacitance Cpara1 and/or Cpara2 may be present at the junction area. The density of the doping in the lightly doped areas will affect the amount of parasitic capacitance Cpara1 and/or Cpara2 created. Therefore, the total capacitance at the gate 384/drain diffusion area 395 may be approximately equal to Cgd+Cpara1, while the total capacitance at the gate 384/source diffusion area 391 may be approximately equal to Cgs+Cpara2.

The presence of parasitic capacitance values Cpara1 and/or Cpara2 may alter the gain of the sensor 300. In a pixel having an chemically-sensitive sensor (e.g., an ISFET) and a row selection transistor, the pixel may be read out in a source follower configuration or in a common source configuration. The parasitic capacitance values may affect the gain of the pixel in different ways depending on which configuration is used. In the source follower configuration, a gain of unity (1) is the maximum gain. Parasitic capacitors serve to attenuate the gain. This is because the signal at the fluidic interface is capacitively coupled to the floating gate of the chemically-sensitive sensor. The parasitic capacitors create a capacitive divider that reduces the charge to voltage conversion that occurs at the gate. Therefore, in source follower configurations, eliminating LDD regions and minimizing the parasitic capacitance provides the largest gain. In the common source configuration, it is desirable to control the parasitic capacitance in order to create negative feedback to establish systematic gain values. Without any parasitic capacitance, the pixel would operate open-loop during readout with a very large gain that is not well controlled by process parameters. Therefore, because LDD regions can be well controlled and consistently matched between devices, the gain can be controlled by these established capacitance values. In the common source configuration, the most important overlap capacitor is Cgd. The gain of the pixel is roughly equal to the double-layer capacitance divided by Cgd. By way of example, if the double layer capacitance is 3 fF and the value of Cgd is 0.3 fF, the gain of the pixel is approximately 10. This becomes a well controlled parameter when it is controlled by the LDD. To decrease the gain, Cgd is increased with larger LDD extensions. To increase the gain, the LDD extensions are reduced to lower Cgd. It is not desirable to have Cgd so small that it is not well controlled. Therefore, controlling the LDD region to achieve a gain in the range from 1 to 20 is desirable.

For example, in another embodiment, the lightly doped region 394 associated with the drain diffusion area 395 may be eliminated, and only the lightly doped region 392 may be present in a sensor 300. In this case, the capacitance attributed to the source terminal 391 may be equal to Cgs+Cpara2, while the capacitance attributed to the drain terminal 395 may only be equal to Cds. The gain of the sensor 300 in this embodiment is different from the embodiment described above in which both parasitic capacitances Cpara 1 and Cpara 2 were present. Accordingly, the addition of lightly doped regions 392 and/or 394 may be used to alter the gain of the sensor 300, and thereby eliminate additional circuitry that may be needed to amplify sensor 300 signal.

An exemplary illustration of a structure of a chemically sensitive sensor according to an embodiment of the present invention may be seen in FIG. 4. The chemically sensitive sensor 400 may include a microwell portion 401, a built-up portion 403, and a substrate portion 405. The microwell portion 401 may include a microwell 410 that may have a passivation layer, such as oxide layer 415, near the bottom of the microwell 410. A chemical reaction to be detected may occur in the microwell 410 and be detected by the a floating gate electrode 420 in the built-up portion 403.

The built-up portion 403 may include the floating gate electrode 420 built upon alternating layers of insulation and dielectric 432, 434, 436, 461, 465, 469; conductive and metal layers 430, 452, 454, 456, 471, 475 and 479; and a gate electrode 484. The substrate portion 405 may include a substrate 499 that may be doped with a P+ dopant, an epitaxial layer 497 that may also be a P-type dopant, and N+ doped regions 491 (source) and 493 (drain). The N+ doped regions 491 (source) and 493 (drain) may be highly doped regions, and the areas labeled 491′ and 493′ may be lightly doped regions. The channel 494 may become conductive based on the signal applied from floating gate 420 to the gate 484. Of course, the doping in the substrate 405, in the regions 491 and 493, in the epitaxial layer 497 and in gate 484 may be reversed.

The chemically sensitive sensor 400 may have, depending upon the mode of operation, a signal gain that is dependent, in part, upon the additional parasitic capacitance provided by the lightly doped regions 491′ and/or 493′ (if present). Embodiments of the chemically sensitive sensor 400 can either be an NMOS or PMOS device, e.g., formed as a standard NMOS or PMOS device with a microwell above a floating gate. The microwell structure 410 may contain an oxide or other material 415 that may transport a chemical sample, e.g., a specific ion(s), to sense a charge on the floating gate 420 of the chemically sensitive sensor 400. This charge transfer may then be read by read circuits (not shown) coupled to the chemically sensitive sensor 400 and the amount of charge transfer may represent an amount of ions contained within the sample in the microwell 410. It is in this way that each chemically sensitive sensor 400 in an array can be used to detect local variations in the sample in the microwell 410, for example, an ion concentration of a sample liquid, that is presented over an array (not shown) of chemically sensitive sensors 400.

Another exemplary structure of a chemically sensitive sensor according to another embodiment of the present invention will be described with reference to FIG. 5. The chemically sensitive sensor 500 may include a microwell portion 501, a built-up portion 503, and a substrate portion 505. The microwell portion 501 may include a microwell 510 that may have a passivation layer, such as oxide layer 515, near the bottom of the microwell 510. The chemical reaction to be detected may occur in the microwell 510 and be detected by the a floating gate electrode 520 in the built-up portion 503.

The built-up portion 503 may include the floating gate electrode 520 built upon alternating layers of insulation and dielectric 542, 545, 546, 561, 565, 569 and conductive and metal layers 552, 555, 556, 571, 574, 575, 577 and 579. A gate electrode 584 and contacts 564, 567 can be formed for the electrodes 581 and 585. The substrate portion 505 may include a substrate 599 that may be doped with a P+ dopant, an epitaxial layer 597 that may also be a P-type dopant, and N+ doped regions 591 (source) and 595 (drain). Of course, the doping in the substrate 505 may be reversed. The electrodes 581 and 585 may accumulate charge from the gate electrode 584 to facilitate confinement and isolation. The charge coupling of the electrodes 581 and 585 with the gate electrode 584 may form a pixel that may be placed into an array for addressable readout. The transistor gain may be increased by the charge transfer from the gate electrode 584 to the electrodes 581 and 585. Furthermore, the charge transfer may be affected by the manipulation of the parasitic capacitance as explained above with respect to FIG. 3, which also affects the transistor gain. The parasitic capacitance may be manipulated by the addition of lightly doped regions 591′ and/or 595′, which may be doped at a lesser density than the highly doped regions 591 and 595. In addition, charge transfer may be also affected by the VR terminal 563 and the Tx terminal 585 that may act as barriers or wells for the charge packets.

It should be noted that embodiments using more or fewer metal and insulation layers are envisioned and the foregoing embodiments are simply exemplary examples. Additionally, the number of electrodes can vary greatly with differing embodiments. Following the formation of the floating gate electrode, any additional electrodes and contacts for the electrodes, the chemically sensitive sensors, including ions, can be formed by creating insulating or dielectric layers out of tetraethyl orthosilicate (TEOS) or oxides and etching microwells above the floating gate electrodes. The microwells can then have a passivation layer placed in at least the bottom of the microwell. Using the techniques described below, chemicals and ions can be sensed using structures made from processes similar to those described above. Varying embodiments of the foregoing structures are possible. For example, the gate electrodes may be formed using single layer polysilicon. The structure can be made using N+ or P+ electrodes. It is also envisioned that embodiments using single electrode polysilicon gap spacing makes it possible to charge couple the electrodes at a small enough process nodes such as 0.13 um and below. Process nodes 0.13 um and below enable a charge coupled structure to work in current CMOS processes. However, it should be noted that that process nodes are not limited to being this small, and can easily be larger. Embodiments employing surface channels, buried channels and using ion implantation to form channels stops using are also envisioned.

Additionally, embodiments using buried charge transfer with multiple N-type implants to create desired potential profiles to avoid interface states and avoid flicker noise are envisioned.

It should be noted that the previous description provides simply exemplary embodiments and that varying processes for the fabricating the chemical sensitive sensor disclosed herein will be readily apparent to those skilled in the art. For example, the lightly doped drains could be formed before the gates and electrodes using masking techniques. Accordingly, the steps discussed herein do not need to be performed in any particular order and may be performed using any semiconductor technique known within the art.

Several embodiments of the present invention are specifically illustrated and described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings. In other instances, well-known operations, components and circuits have not been described in detail so as not to obscure the embodiments. It can be appreciated that the specific structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments.

Those skilled in the art may appreciate from the foregoing description that the present invention may be implemented in a variety of forms, and that the various embodiments may be implemented alone or in combination. Therefore, while the embodiments of the present invention have been described in connection with particular examples thereof, the true scope of the embodiments and/or methods of the present invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.

Various embodiments may be implemented using hardware elements, software elements, or a combination of both. Examples of hardware elements may include processors, microprocessors, circuits, circuit elements (e.g., transistors, resistors, capacitors, inductors, and so forth), integrated circuits, application specific integrated circuits (ASIC), programmable logic devices (PLD), digital signal processors (DSP), field programmable gate array (FPGA), logic gates, registers, semiconductor device, chips, microchips, chip sets, and so forth. Examples of software may include software components, programs, applications, computer programs, application programs, system programs, machine programs, operating system software, middleware, firmware, software modules, routines, subroutines, functions, methods, procedures, software interfaces, application program interfaces (API), instruction sets, computing code, computer code, code segments, computer code segments, words, values, symbols, or any combination thereof. Determining whether an embodiment is implemented using hardware elements and/or software elements may vary in accordance with any number of factors, such as desired computational rate, power levels, heat tolerances, processing cycle budget, input data rates, output data rates, memory resources, data bus speeds and other design or performance constraints.

Some embodiments may be implemented, for example, using a computer-readable medium or article which may store an instruction or a set of instructions that, if executed by a machine, may cause the machine to perform a method and/or operations in accordance with the embodiments. Such a machine may include, for example, any suitable processing platform, computing platform, computing device, processing device, computing system, processing system, computer, processor, or the like, and may be implemented using any suitable combination of hardware and/or software. The computer-readable medium or article may include, for example, any suitable type of memory unit, memory device, memory article, memory medium, storage device, storage article, storage medium and/or storage unit, for example, memory, removable or non-removable media, erasable or non-erasable media, writeable or re-writeable media, digital or analog media, hard disk, floppy disk, Compact Disc Read Only Memory (CD-ROM), Compact Disc Recordable (CD-R), Compact Disc Rewriteable (CD-RW), optical disk, magnetic media, magneto-optical media, removable memory cards or disks, various types of Digital Versatile Disc (DVD), a tape, a cassette, or the like. The instructions may include any suitable type of code, such as source code, compiled code, interpreted code, executable code, static code, dynamic code, encrypted code, and the like, implemented using any suitable high-level, low-level, object-oriented, visual, compiled and/or interpreted programming language. 

What is claimed is:
 1. A method of making a chemically sensitive sensor, comprising: forming a substrate with a first conductivity type of dopant; building an epitaxial layer using the same conductivity type dopant used to form the substrate, but made less dense than the dopant on the substrate; forming an electrode layer on the epitaxial layer formed from a different, second conductivity type of dopant than the first conductivity type of dopant used to form the substrate, wherein the density of dopant on both the electrode layer and the substrate are similar; masking and etching the electrode layer to produce gates and corresponding electrode pairs; creating a first lightly doped region adjacent to one electrode of each of the electrode pairs using a multidirectional implant technique, wherein the first lightly doped region is formed from a dopant of a conductivity type opposite the epitaxial layer dopant; producing diffusion nodes that are self-aligned with the electrode pairs, a first of said diffusion nodes contiguous with the first lightly doped region, from a dopant of a conductivity type similar to the gates, electrode pairs, and lightly doped region; and forming a floating gate electrode, and contacts for the electrode pairs by alternating layers of insulation, dielectric, conductive and metal layers.
 2. The method of claim 1, further comprising: creating a second lightly doped region adjacent to the other electrode of each of the electrode pairs using a multidirectional implant technique, wherein the second lightly doped region is formed from a dopant of a conductivity type opposite the epitaxial layer dopant.
 3. The method of claim 1, wherein a second of said diffusion nodes is contiguous with a second lightly doped region.
 4. The method of claim 1, further comprising: forming an additional electrode on the diffusion nodes.
 5. The method of claim 1, further comprising: forming a microwell to hold a sample above the floating gate. 